The present invention relates generally to a means and method for stabilizing shorelines. More specifically, the invention relates to an offshore barrier module and its use for stabilizing an adjacent shoreline.
Waves impinging on a beach dislodge soil particles, i.e. sand, which are then swept seawrd where they can form an underwater ridge of sand parallel to the shoreline. This is known as a long shore bar. In times of heavy wave action, the beach tends to decrete, i.e. lose sand, and the bar tends to accrete. In times of light wave action, however, the movement of sand particles is reversed and the beach tends to accrete at the expense of the bar. This is a natural phenomenon and on most beaches there is a constant exchange of sand between these two features with the direction of transport being dependent on the character of the waves. When the waves are large and follow close upon each other, as they do under storm conditions, the beach is eroded and the bar builds up. When calm conditions return, the small waves rebuild the beach at the expense of the bar.
However, this natural beach stabilizing phenonmenon is not always present because of littoral currents, predominating wind directions, man-made objects which are interposed on the beach or other natural and artificial causes. Various structures have therefore been utilized in an attempt to stabilize beaches.
It has long been known that beaches and shorelines of rivers, lakes, and oceans may be protected or stabilized by placing structures along the shoreline or at some distance offshore under water. The structures can serve to obstruct the flow of sediment carrying water and cause the deposition of at least part of the sediment in the immediate vicinity of the stabilizing structure. Examples of such structures are impermeable groins, permeable groins, artificial seaweed and the like.
Impermeable groins are constructed of sheet piling of steel, concrete, or wood driven in a continuous row generally perpendicular to the shoreline. Sediment will be deposited from water moving transverse to the row of piling in a direction upstream from the structure. Such groins may also be constructed from mounds of stones, concrete blocks, and the like.
Permeable groins are similarly constructed except that the sheet piles are driven at some distance apart permitting sediment carrying water to pass through the structure between the sheets but so reducing the velocity of the water which passes through that heavier particles of sediment will be deposited downstream from the structure. Such structures are constructed by securing vertical boards at some distance apart on a frame and positioning the frame transverse to the flow of sediment carrying water where it is secured by piles driven into the ground.
Another type of stabilizing structure is artificial seaweed. It has been discovered that naturally growing seaweed tends to trap water-borne sediments by reducing the water velocity to the point that sediment deposition occurs. This effect is duplicated by artificially assembling clusters of low density synthetic tapes or filaments and securing the clusters to the seabottom by a weighting means such as sandbags to form an array of some area in extent similar to a large bed of naturally growing seaweed. Such arrays have been used to protect the legs of offshore oil drilling platforms from erosion of the surrounding soil. Synthetic filaments have been produced from air blown polypropylene to produce filaments of maximum buoyancy.
However, all of the aforementioned types of barrier structures have their problems. Impermeable groins are extremely expensive to construct and obstruct the full utilization of the water adjacent the shore by small boat traffic and individuals. Also, for construction purposes, access to the site is required by heavy equipment. Soil decretion downstream or downdrift of such groins is pronounced and is detrimental to the owners of adjacent property.
Permeable groins present all of the problems mentioned with regard to impermeable groins except that they cause a downdrift accretion of sediment and so may actually benefit the owners of downdrift property. However, permeable groins are ineffectual in preventing a littoral movement of soil particles dislodged by wave action unless they extend so far seaward from the shoreline as to be impossibly expensive. Thus, permeable groins are used effectively only to control the movement of soil parallel to the shoreline caused by longshore currents as in a river or along a shoreline where there is a pronounced littoral sediment carrying current.
Artificial seaweed has been successfully used at substantial depths of water to trap current borne sediment. However, it has not been effective in preventing the movement of sediment along shorelines. This is so because the high velocity and turbulence of water near a shore due to the breaking of waves causes the synthetic filaments to assume a position nearly parallel to the flow of water and so the water-borne sediments easily pass over the filaments with a minimal reduction in water velocity. Also, the plastic from which the filaments are constructed apparently suffers a loss of buoyancy with the passage of time which then renders the filaments ineffective to trap sediment.
Another known apparatus for prevention of shore erosion involves the use of modules including a plurality of spaced wooden boards secured to belts which are anchored to a concrete base. This assembly is said to cause sand to be deposited from de-energized waves. However, the modules are not commercially acceptable because the floating wooden boards are objectionable to bathers and others who wish to use the adjacent shoreline. Since the wooden boards are rigid, bathers can be struck and injured by them as the boards are moved by the action of waves in the body of water.
Accordingly, it has been considered desirable to develop a new and improved means and method for stabilizing shorelines which would overcome the foregoing difficulties and others while providing better and more advantageous overall results.